A power storage system that is equipped with a secondary cell pack normally includes a protection function that protects the secondary cell in a secondary cell pack from abnormal states such as overcurrent, over-discharging, and overcharging.
For example, in a power storage system that uses, for example, conventional lead storage batteries as secondary cells, a system protection unit such as a power-supply control unit that is outside the secondary cell pack and that controls the charging and discharging of the secondary cells has a system protection function of detecting abnormal states of the secondary cells and cutting off the electrical connection between the secondary cells and the outside of the power storage system (refer to Patent Document 1).
In a power storage system that uses the recently developed lithium-ion secondary cells as secondary cells, a cell protection function is provided as a protection function in the secondary cell pack itself due to the necessity to monitor, for example, the voltage of each lithium secondary battery cell in order to detect abnormal states and due to the possibility that large currents may flow through the secondary cells. In the cell protection function, the cell protection unit that is provided in the secondary cell pack detects abnormal states of the secondary cells and cutting off the electrical connection between the secondary cells and the outside of the secondary cell pack (refer to Patent Document 2).